In order to rotatably support a wheel with respect to the suspension system, there are used various kinds of drive wheel rolling bearing units in which an outer ring and an inner ring are rotatably combined together through rolling elements. For example, in Japanese Unexamined Patent Publication No. 2003-139174, a drive wheel rolling bearing unit 1 as shown in FIGS. 23 and 24 is described. This drive wheel rolling bearing unit 1 comprises an outer ring 2, and a hub 3 and an inner ring 4 which are rotatably supported on the inner diameter side of the outer ring 2 through a plurality of rolling elements 5. Out of these members, the outer ring 2 is securely connected to a knuckle 7 constituting a suspension system by the use of a first flange 6 provided on the outer peripheral surface thereof, and is not rotated even when used. Moreover, on the inner peripheral surface of the outer ring 2, there are provided double row outer ring raceways 8, so as to rotatably support the hub 3 and the inner ring 4 on the inner diameter side of the outer ring 2 concentrically with the outer ring 2.
The hub 3 is provided with a second flange 11 for supporting a wheel 9 constituting a vehicle wheel, and a disc 10 constituting a disc brake, on a portion towards the axial outside end of the outer peripheral surface (“outside” in relation to the axial direction, is the outside in the widthwise direction of the vehicle in a condition when assembled in the vehicle; the left side in the figures. The same applies throughout to this specification and claims). The wheel 9 and the disc 10 are fixedly connected to the outside surface of the second flange 11 by means of a stud 12 and a nut 13. A first inner ring raceway 14 is formed in a middle portion on the outer peripheral surface of the hub 3. The inner ring 4 which is formed with a second inner ring raceway 16 on the outer peripheral surface thereof, is externally fitted with an interference fit to a small diameter stepped portion 15, which is a fitting cylindrical surface portion and is formed on the axial inside end (“inside” in relation to the axial direction is the side towards the widthwise center when assembled in the vehicle; the right side in the figures. The same applies throughout to this specification and claims).
In the center of the hub 3, a spline hole 17 is provided. A spline shaft 19 constituting a constant velocity universal joint 18 is inserted into the spline hole 17 from the axial inside towards the outside. A nut 21 is screwed onto a male screw portion 20 which is provided in a portion protruded beyond the outside end surface of the hub 3 on the axially outside end portion of the spline shaft 19, and is further tightened, so that the constant velocity universal joint 18 and the drive wheel rolling bearing unit 1 are securely connected to each other. In this condition, the axial inside end surface of the inner ring 4 is abutted against the axial outside end surface of a constant velocity universal joint outer ring 22 constituting the constant velocity universal joint 18, so that the inner ring 4 is not displaced in a direction to come away from the small diameter stepped portion 15. At the same time, an appropriate preload is applied to the respective rolling elements 5.
The shapes, and the like, of the components of the constant velocity universal joint 18 are similar to those of a known constant velocity joint of a Rzeppa type or a Birfield type, and have nothing to do with the gist of the present invention. Therefore, detailed illustration and description are omitted. Moreover, as shown in FIG. 25, a structure where a cylindrical portion 23 existing on a portion protruded axially inside beyond the inner ring 4 which is fitted onto the small diameter stepped portion 15a on the axial inside end of the hub 3a is swaged and expanded (plastically deformed) radially outward to form a crimped portion 24, and the inside end surface of the inner ring 4 is clamped by the crimped portion 24, is described in Japanese Unexamined Patent Publication No. 2003-139174 and the like, and is heretofore well known. In the case of such a structure, an appropriate preload is applied to the respective rolling elements 5 in a condition with the inside end surface of the inner ring 4 clamped by the crimped portion 24.
The operations for forming the spline hole 17 in the center of the hubs 3 and 3a, and assembling the respective components, have been heretofore performed as follows, either for the case of the structure to clamp the inside end surface of the inner ring 4 by means of the constant velocity universal joint outer ring 22 as shown in the FIGS. 23 and 24, or for the case of the structure to clamp the inside end surface of the inner ring 4 by means of the crimped portion 24 as shown in the FIG. 25. The following description mainly focuses on the structure shown in FIGS. 23 and 24. Firstly, as shown in FIG. 26, before assembling the respective components including the inner ring 4 into the hub 3 (3a), a female spline portion for constituting the spline hole 17 as shown in FIG. 27 is formed by applying broaching to the inner peripheral surface of an original hole 25 which is provided in the center of the hub 3 (3a). Next, as shown in FIG. 28, in a condition with the respective components, excluding the inner ring 4, assembled into the hub 3 (3a), then as shown in FIG. 29, the inner ring 4 is externally secured to the small diameter stepped portion 15 which is provided on the axial inside end of the hub 3. Then, after forming the crimped portion 24 (refer to FIG. 25) if necessary, the drive wheel rolling bearing unit 1 and the constant velocity universal joint 18 are assembled to each other by inserting the spline shaft 19 constituting the constant velocity universal joint 18, into the spline hole 17 as shown in FIG. 23. In this manner, conventionally, the operation for forming the spline hole 17 in the center of the hub 3 (3a) has been performed before assembling the inner ring 4 into this hub 3 (3a).
However, if in this manner the inner ring 4 is externally secured to the small diameter stepped portion 15 after the processing for the spline hole 17, it is unavoidable that the axial inside end of the spline hole 17 is contracted radially inward although slightly. That is, since the inner ring 4 is externally secured to the small diameter stepped portion 15 (with the inner diameter A of the inner ring 4 in the free state< the outer diameter B of the small diameter stepped portion 15 in the free state), a large force directed radially inwards acts around the whole periphery of the small diameter stepped portion 15 which is relatively thin when fitted. Then due to such a force, the axial inside end of the spline hole 17 is deformed radially inward as shown exaggerated in FIG. 29, and the diameter of the inner peripheral surface of the axial inside end of the spline hole 17 is contracted (the inner diameter of the inscribed circle of the female spline grooves and female spline teeth and the overpin diameter are contracted). When the diameter of the inner peripheral surface of the axial inside end of the spline hole 17 is contracted in this manner, it becomes difficult to insert the spline shaft 19 into the spline hole 17, and the assembly operation for the drive wheel rolling bearing unit 1 and the constant velocity universal joint 18 becomes difficult to perform.
In order to avoid this inconvenience, Japanese Unexamined Patent Publication No. 2002-317824 discloses an invention wherein a female spline portion for constituting the spline hole 17 is formed in a condition where the inner ring 4 or a jig having an equivalent inner peripheral surface, is externally secured to the small diameter stepped portion 15. That is, by externally securing the inner ring 4 or the jig to the small diameter stepped portion 15 in this manner, the axial inside end of the spline hole 17 is elastically deformed in the same condition as where the inner ring 4 is externally fitted tightly to the small diameter stepped portion 15. Then, by forming the female spline portion in this condition, the female spline portion is formed with compensation for the deformation which is caused by externally fitting tightly the inner ring 4 to the small diameter stepped portion 15. Therefore, the difficulty of performing the assembling operation for the wheel drive rolling bearing unit 1 and the constant velocity universal joint 18, caused by the difficulty of inserting the spline shaft 19 into the spline hole 17 can be avoided.
However, in the case of forming the female spline portion in a condition with the inner ring 4 or the jig externally fitted to the small diameter stepped portion 15 in this manner, it is necessary to take off the inner ring 4 or the jig from the small diameter stepped portion 15 after forming the female spline portion 4. Such a procedure of taking off the inner ring 4 from the small diameter stepped portion 15 may cause damage to the respective peripheral surfaces due to rubbing between the inner peripheral surface of the inner ring 4 and the outer peripheral surface of the small stepped portion 15 because this inner ring 4 and the small stepped portion 15 are tightly fitted to each other. By forming the female spline portion in the center of the hub 3 in the condition where at least the outer ring 2, the respective driving elements 5, and the inner ring 4 are assembled into the hub 3, it becomes unnecessary to take off the inner ring 4 from the small stepped portion 15. However, when forming the female spline portion in the condition where the respective components 2, 4, and 5 are assembled in this manner, treatment to prevent cutting scraps caused by the process of forming the female spline portion, from entering the gap between the respective components 2, 4 and 5 becomes necessary, and an operation to rinse out these cutting scraps becomes more troublesome. Therefore this method is undesirable.
Moreover, as shown in the FIG. 22, in the abovementioned structure where the crimped portion 24 is formed on the inside end of the small diameter stepped portion 15a, Japanese Unexamined Patent Publication No. 2002-327715 discloses an invention to avoid the difficulty of inserting the spline shaft 19 into the spline hole 17 due to contraction of the inner diameter of the axial inside end of the spline hole 17 caused by the formation of the crimped portion 24. That is, in the case of the invention described in Japanese Unexamined Patent Publication No. 2002-327715, as shown in FIG. 30, by externally fitting tightly a jig 26 to the small diameter stepped portion 15a, the inner peripheral surface of the axial inside end of the spline hole 17 is elastically deformed in the same condition as when the crimped portion 24 (FIG. 22) is formed. Then, by forming the female spline portion for constituting the above spline hole 17 in this condition, this female spline portion is formed in a condition with compensation for the deformation caused by forming the crimped portion 24.
Also for the invention described in this Japanese Unexamined Patent Publication No. 2002-327715, it is necessary to take off the jig 26 from the small diameter stepped portion 15a after forming the female spline portion, similarly to the above invention described in Japanese Unexamined Patent Publication No. 2002-317824. Moreover, in the case of the invention described in Japanese Unexamined Patent Publication No. 2002-327715, since the inner peripheral surface of the axial inside end of the spline hole 17 is elastically deformed in the same condition as when the crimped portion 24 is formed, the interference between the inner peripheral surface of the jig 26 and the outer peripheral surface of the small diameter stepped portion 15a becomes greater. Therefore, damage of the outer peripheral surface of the small diameter stepped portion 15a becomes greater, and can be too much to ignore.